I. Technical Field
The present invention relates to drilling and logging, particularly for exploiting hydrocarbons from underground formations.
More specifically, the present invention relates to an assembly for drilling and logging, a method for drilling and logging and a device for electro pulse drilling particularly feasible for working the method.
II. Description of the Related Art
Most deposits of gas and/or liquid hydrocarbons are found hidden inside the underground in strata of rock sediments. Drilling into the underground to be able to produce hydrocarbon fluids from a hydrocarbon-containing formation is very costly. To increase the possibility for the well to hit a hydrocarbon containing reservoir, the underground is mapped as much as possible before drilling. Seismic techniques, and to a lesser extent electromagnetic techniques are used for the mapping.
But also during drilling there is significant interest in having information on the underground in order to be able to better plan and control the drilling operation so that assumed or identified hydrocarbon reservoirs can be produced. Advantageously, the information is as detailed and correct as possible, for improved planning and control of the drilling operation.
Drilling is usually performed with a drill string that is rotated from the surface. An alternative method is to use a down-hole motor near the drilling bit, for rotation thereof. The motor can, for example, be driven by the circulating drilling mud.
A third method for drilling is electro pulse drilling, so-called EPD-Electro Pulse Drilling. When electro pulse drilling, powerful electric pulses are used to break up the formation, instead of a pure mechanical treatment as used by the traditional drilling processes. More specifically, at least two electrodes that are separated with an insulating material are used, one electrode being charged to a high electrical potential, while the other electrode is grounded. At sufficiently high charging, a powerful spark will be formed as the electrodes are short-circuited by the current going the way of lowest resistance from the charged electrode to the grounded electrode. A drill bit for electro pulse drilling comprises at least the two types of electrodes, in different configurations, and may in addition comprise one or several additional electrodes that during operation will take potential between the electrodes, so-called floating electrodes. There is a requirement for high voltage, typically several kilovolts, and high current to deliver sufficient energy to break up the formation ahead of the drilling bit where the electrodes are arranged. To have sufficient effect, at power-full electric supply is required, typically comprising a number of elements for charging and subsequent discharging of the electrical energy, which elements typically are in the form of condensers. Dependent on the formation and the operational parameters, the electrical spark will form a pressure wave in the drilling fluid ahead of the drilling bit, or form a plasma channel in or at the formation, in both circumstances resulting in breaking up of the formation.
Electro pulse drilling is described in several patent applications, of which three to are described below.
In patent publication U.S. Pat. No. 4,741,405 a device is described for focused electro pulse drilling, comprising multiple electrodes. The drill bit includes at least a first electrode that is positively coupled to a pulse-forming line, a second electrode that is coupled to the ground potential, and a number of third electrodes that are arranged near the first and the second electrode, the third electrodes being floating electrodes. By charging the first electrode, at the correct conditions a spark will be formed between the first electrode and one of the third electrodes, further to a next of the third electrodes, until the spark finally goes over to the grounded second electrode. The drill bit is against the formation surrounded by a working fluid, and the sparks form a pressure wave in the working fluid directed at the material to be drilled out. It is assumed that in the working fluid plasma channels are formed, resulting in pressure waves, which again have sufficient energy to break down the material to be drilled out. By a sophisticated switched coupling of the pulse-forming lines between an electric supply and selected first electrodes in the drill bit, a focusing of the pressure wave can be achieved, such that the drill can be controlled and directed. The high-voltage power can be delivered to the pulse-forming lines either down-hole or from the surface. A number of means to this end are described, amongst them use of a Marx-generator as high-voltage source. Further, it is described for example in column 4, lines 32-46 that the electro pulse drill as such can be used as a source for seismic impulses to determine the position of the drilling bit and thereby the position of the well-bore. More specifically, pressure waves generated by the drill bit are used as seismic impulses that can be received by a network of seismic receival units positioned on the surface close to the well-bore. Seismic time-distance data can be real-time processed, and a continuous presentation of the well-bore position and deviations from a normal can be projected by use of a dedicated unit. The deviations can then be used to correct and direct the drilling device by generating control signals from the surface. In the patent publication there is only a short description of seismic time/distance analysis to determine the position, nothing is mentioned about mapping formation properties based on the received seismic data or other data.
In patent publication RU 2167991 C2 electro pulse drilling is described in the form of rotational drilling with a mechanical tool, with high-voltage electric pulses traveling over the bottom of the well-bore to reduce the formation strength. Thereby more effective drilling is achieved.
In patent publication WO 03/069110 A1 electro pulse drilling is described in the form of plasma channel drilling. More specifically, an apparatus and a method are described, using a pulsed high-voltage generator coupled to an electrode assembly placed at a material removal station for the apparatus, which means down into a well-bore, the apparatus being adapted to generate electrical pulses to form a plasma channel repetitively per second within or on a surface of a material, so that material is removed by rapid expansion of each plasma channel that fractures and fragments the material body. 1-100 pulses per second are used, typically 5-25 pulses per second, lasting 1-50 micro seconds, and rise time below 150, typically below 100 nanoseconds, with high-voltage in the range 10-50 kV and effect with top value 1-100 MW. Voltage is limited is to about 50 kV to avoid too comprehensive requirements as to insulation. Holes can be drilled with diameters up to 100 mm. Drilling in a “dry” well-bore is also possible, see page 6, line 22, but preferably a drilling fluid of low electric conductivity is used, such as water, which is preferable for removal of fragments. The electrodes are concentrically arranged, with ground outermost. The publication provides a good and comprehensive description of physical parameters in connection with electro pulse drilling. The significance of the dielectrical properties of fluid and formation ahead the drilling bit is described relative to different operating parameters. Inter alia, it is described that at relative high voltage, the electrical break-down will take place in the formation instead of in fluid surrounding the drilling bit. The FIGS. 6 and 7 illustrate break-down of the insulating dielectricum (either the formation that is drilled out or fluid surrounding the drilling bit) as function of voltage or voltage field, and time for discharging. Likewise, the significance of distance between the electrodes is illustrated and described. On page 16, lines 7-21, for operating parameters resulting in the electrical break-down in the formation instead of in a fluid, it is explained how the plasma channels gradually will break down the formation over the whole area of the drilling bit in the direction of drilling, because the plasma channel seeks to go the way of lowest resistance, which way thereby will be the formation. The plasma channel formation will over time rotate 360° around the drilling bit face, whereby material ahead of the full drilling bit is broken down and there is no need to rotate the drill bit with electrodes per se. Further, it is described that the pulse generator preferably can generate a high-voltage pulse with a wave-form near the critical damped response from the circuit formed when the plasma channel is produced. Formation of a plasma channel results in break-down of the dielectrical resistance, why next plasma channel formation should wait until the dielectrical resistance is reestablished. In the publication there is no discussion of electro pulse drilling and simultaneous logging of any kind.
Measurement while drilling, MWD, means to collect data on pressure, temperature and path of drilling in three dimensions during or in connection with drilling. Measurements while drilling to map the formation itself is usually termed logging-while drilling, LWD, which includes measurement of parameters as resistance, porosity, acoustic velocity and gamma radiation. In Patent Publication U.S. Pat. No. 5,881,310 a closer description of LWD and MDW is found. In said publication it is described that the drilling string and drill bit can be used as a source for acoustic signals, but electro pulse drilling is not mentioned.
In Patent Publication WO 2004/083898 A1 a method and an apparatus are described for determining the properties of underground reservoirs by using an electromagnetic transmitter and a seismic transmitter with in substance identical location on the ground or seabed, which transmitters generate signals that are received by an electromagnetic receiver and a seismic receiver, said receivers having in substance identical location on the ground or seabed, separated from the transmitters. Preferably low acoustic and electro magnetic frequencies are used, in the range 0.1 to 20 Hz, to have reach. By analyzing and comparing different components of electromagnetic field and acoustic response, particularly refracted components, data indicating presence of a hydrocarbon reservoir can be identified and analyzed.
In Patent Application WO 02/14906 A1 a method and an apparatus is described for determining properties of underground reservoirs. An electromagnetic field is applied from the seabed by use of a dipole antenna-transmitter, which field is detected by using a dipole antenna-receiver. Measurements are taken with the antenna-receiver both in-line and parallel and the difference between the two sets of measurements give indication of interest. Dependent on the angle of incidence and state of polarization an incident electromagnetic wave to a layer of high resistance may excite a ducted or guided wave mode in the layer. Such high-resistive layer can be a hydrocarbon-containing layer. The ducted wave modus propagate laterally along the layer and leaks energy back to the overburden and receivers positioned on the seabed. The ducted wave-modus is excited only for an incident wave with transverse magnetic (TM) polarization, which means magnetic field perpendicular to plane of incident, and at angles of incidence close to the Brewster angle and the critical angle (the angle of total reflection). For electromagnetic waves with transverse electric polarization (TE) the ducted mode will not be excited. The transmitter generates both TE and TM waves, but by varying the orientation of the transmitter, optionally using orthogonally oriented receiver antennas, the two modes of waves can be received at different sensitivity.
The above-mentioned methods for mapping the underground are comprehensive and expensive. Often the signals are noisy due to interference and interaction, inter alia, with other sources, long path of the signal, and problems with weak contact with the seabed and disturbing effects of surrounding seawater. There is a demand for better technology for logging, particularly at simultaneous drilling, and particularly so that comprehensive quantities of data of preferably independent data can be collected. There is also demand for a device for electro pulse drilling that is particularly feasible for simultaneous electro pulse drilling and generation of signals for logging.
The objective of the present invention is to meet the above demands.